Switching power supplies have long been of great interest to product designers because of their compact size relative to their linear counterparts. But, it was not until the second half of the 1980's that switching power supplies (i.e., "switchers") became the power supply of choice in the design of most electronic equipment. Their increased popularity was largely due to the availability of switchers that were more compact, lighter in weight, equally as reliable, yet only slightly more expensive than linear designs of equivalent power rating.
The key to the appearance of highly reliable, compact switcher designs was the availability of high-frequency switching transistors that could withstand the high voltage transients which appear on AC mains. With the development of FET's and other types of fast switching transistors that could operate reliably in the AC mains environment, off-line type switching power supplies, designed around small transformers, became practical. Thus, the large, 50 and 60 Hertz, iron core transformers that were required in the classical linear power supplies were replaced by higher-frequency transformers that greatly reduced their size and weight. Consequently, the switching power supplies of today are smaller, lighter in weight and more efficient than previous linear designs.
However, with the constant push to miniaturize electronics products there is a never-ending demand for even smaller and lighter power supplies This translates into a demand for smaller transformers, since the transformer is still the largest and heaviest component, even in today's switching power supplies.
It is well understood that small transformers are quite realizable for use at MegaHertz frequencies. However, the transformer in an off line switcher must operate in the AC mains environment. This means that there are stringent isolation requirements which any such transformer design must satisfy. Since isolation is largely an issue of the separation and insulation between wires, windings, layers of windings and connections, it is apparent that the isolation requirements work against minimizing size. This trade-off has significant quality control. inspection and cost implications.
One of the most promising techniques for designing small, high-frequency, transformers is the low-profile planar, or printed circuit board (i.e., PCB) style of transformer. In this type of transformer, the primary windings, which are a spiral of traces on a planar surface, are coupled to the secondary windings, which are a different spiral of traces on a separate planar surface, by enclosing the windings in a magnetic housing. Typically, the magnetic housing is made of ferrite, Sumarium or some other composite material that is shaped as a pot-core, an R-M core, an E core, an I core, etc. But, it can be almost any shape that is easy to place around the windings and effectively confines the magnetic field to the area around the windings.
The use of planar traces rather than the classical wire windings on a bobbin is a significant manufacturing advance for high-frequency transformers. However, the international safety standards for interwinding isolation have presented a stumbling block in applying this construction technique to the miniaturization of transformers for off-line switchers. The requirements for isolation necessitate interwinding distances that, before this invention, could only have been addressed by the brute force approach of using thick bobbins, and many layers of insulating spacers. These, though, would not have been efficient transformers because they would have required relatively large magnetic elements, to compensate for the poor coupling between the primary and the secondary windings. Consequently, the inability to satisfy the international safety requirements in a small, light-weight, efficient design has kept low-profile planar transformers out of consumer products, and away from the AC mains. Low-profile planar transformers have been limited to military products, where less isolation is required, and to DC-to DC switchers, where the input is a low DC voltage, not the AC mains. Nevertheless, the real challenge for planar transformers is to be approved for use in consumer oriented off-line switchers. But, in order to be approved for such applications there are specific isolation requirements that they must meet. These are the requirements of the safety certification agencies throughout the world. These agencies define how to measure safety in virtually all consumer products, and these same agencies pass or fail electrical and mechanical products against their published safety specifications.
Almost every country has its own safety agency; however, the most influential and commercially most important among the international agencies are Underwriters' Laboratory (U.L.) in the USA, V.D.E. in Germany, and C.S.A. in Canada. In the case of power transformers that operate at both 110VAC and 220VAC, the U.L., V.D.E, and C.S.A standards that challenge the transformer designer are: (A) the primary winding-to-SELV winding (Safe Extra Low Voltage winding) insulation thickness must be either one insulator that is at least 2 mm (0.080") thick, or three layers of insulation each at least 0.1 mm (0.004") thick (i.e., 3 plys); (B) the "creepage" and "clearance" between the low voltage, secondary winding and either AC line or neutral must be at least 6 mm (0.240"); and (C) the "creepage" and "clearance" between the core and either line or neutral must be at least 2 mm (0.080"). "Creepage" and "clearance" are investigated between conductors, conductors and terminals, grounded or ungrounded conductive parts, components and component leads. "Creepage" is defined as the shortest path between two conductive parts or between a conductive part and the grounding surface of the equipment measured along the surface of the insulation. "Clearance" is the shortest distance between two conductive parts as measured through air. If a barrier is interposed, the spacing is measured around the barrier, or, if the barrier consists of two or more uncemented pieces, the spacing is measured through a joint or around the barrier, whichever is least.
While providing low profile and high-efficiency, PC board (i.e., low profile planar) type transformers for off-line switchers have had difficulty meeting the above requirements.
Therefore, an object of this invention is to provide a low profile planar transformer design and physical construction concept that easily meets the above-stated isolation requirements for use in commercial off line switchers.
It is a further object of this invention to provide an inexpensive to manufacture, low-profile planar transformer with creepage and clearance values that easily meet the VDE specifications while packaged in a small volume and height.
It is yet another object of this invention to provide a bobbin design for a planar transformer that retains the windings in a minimum profile housing while providing the necessary creepage and clearance between the primary and secondary windings.
Another object of this invention is to provide a high-frequency transformer that is useful in consumer applications where it must provide isolation from AC mains.
Another object is to provide a transformer that is the basis for cost-effective consumer-oriented off-line switchers.
It is still a further object of this invention to provide a high frequency transformer that is inexpensive to manufacture.